2.1 Immunoassay Systems
The prior art teaches many immunoassays based on the pioneering development of radioimmunoassay (RIA) by Yalow and Berson, 1960, J. Clin. Invest., 39:1157). RIAs are characterized by competing fixed amounts of radiolabeled analytes with unknown quantities of unlabeled analytes for fixed amounts of specific antibody. The amount of radioactive analyte either bound to antibody or free in solution is quantitated in an appropriate counter and the concentration of non-radioactive analyte determined. Improvements on this general scheme have included: (1) substitution of the radioactive tracer with enzyme or fluorescent tracers, (2) substitution of polyclonal animal antibodies with monoclonal antibodies, (3) improved methods of signal detection including spectrophotometers, fluorometers, fluorescence polarizers and particle counters, and (4) the introduction of homogeneous assays not requiring physical separation of bound tracer from free tracer. Separation of bound tracer from free tracer frequently requires solid supports such as plastic, paper, glass or acrylamide. Customarily antibody is bound to the solid phase whereas tracers and unknowns are free in solution. The bound/free separation is accomplished by one or more washes of the solid phase. The residual bound activity is then measured. These assays are known collectively as heterogeneous immunoassays. In comparison, homogeneous assays obviate the need for the imprecise and time-consuming separation steps.
Commercialization of immunoassays has seen a shift in usage from radioimmunoassays, to enzyme-linked immunosorbent assays (ELISA), to homogeneous assays. This shift is due to the commercial demands of speed, simplicity, automation and absence of radioactivity. Homogeneous assays consist of several types: (1) nephelometry, (2) particle counting, (3) fluorescent quenching, (4) fluorescence polarization, and (5) enzyme assays.
The first nephelometer to measure light dispersion to quantitate immune reactions was devised in the late 1960s. These early nephelometers were improved ten years later with new chemistries, lower angles for measuring dispersion angles and the ability to measure the rate of the antigen-antibody reaction during the first seconds after mixing the reactants (Ritchie, Alper and Graves, 1969, Arthritis Rheum. 12:693; Deaton et al., 1976, Clin. Chem. 22:1465). These assays are of extremely poor sensitivity and are applicable to determinations of analytes at concentrations greater than 10.sup.-8 M, e.g., serum IgE, IgA and IgM levels. In homogeneous particle counting assays, polystyrene particles 0.8 .mu.m in diameter (latex particles) are coated by antibodies. Antigen concentrations can be determined by the concentration of latex particles agglutinated as determined by an instrument capable of distinguishing agglutinated versus non-agglutinated particles (Cambiaso et al., 1977, J. Immunol. Meth. 18:33). Homogeneous fluorescent quenching assays label either antigens or antibodies with a fluorophor. Analyte-antibody-fluorophor complexes yield significantly less fluorescence compared to the antigen-fluorophor or antibody-fluorophor alone (Ullman et al., 1979, J. Biol. Chem. 251:4172; U.S. Pat. Nos. 3,998,943; 3,996,345; 4,174,384; 4,161,515; 4,208,479 and 4,160,016). All these assays involve various methods of quenching fluorescence such that the amount of quenching is related to the amount of the unknown analyte or antibody in the sample. These assays are of low sensitivity (analytes at fluid concentrations greater than 10.sup.-10 M). The low sensitivity is due to endogenous serum fluorescence and the use of fluorescence in a static non-enzymatically amplified manner. Fluorescence polarization assays are based on the free rotation of antigen-fluorophor in solution which is significantly reduced by antibody binding to the antigen-fluorophor and have found considerable commercial success with low molecular weight (under 1000 daltons molecular weight) analytes (Dandliker et al., 1973, Immunochemistry 10:219).
The various immunoassay methods each possess commercial advantages and disadvantages. RIAs are sensitive and easy to set-up but require radioactivity, separation steps and expensive instrumentation. Heterogeneous assays with enzymes or fluorophores eliminate radioactivity and some instrumentation but require separation steps. From a commercial viewpoint it is desirable to eliminate separation steps for several reasons. Separations (1) are labor intensive, (2) are time consuming, (3) require additional equipment, (4) increase variability in results, and (5) preclude high levels of automation. Despite the many commercial advantages of homogeneous immunoassays only three systems, the enzyme-labeled system of Rubenstein et al., U.S. Pat. No. 3,817,837, the substrate-labeled system of Burd et al., 1977, Clin. Chem. 23:1402, and fluorescence polarization (Dandliker et. al., 1973, Immunochemistry) have found commercial success. Yet these three assay systems are limited to small (less than 1000) molecular weight analytes and analytes found in concentrations greater than 10.sup.-10 M.